The invention relates to a ventilator, according to the preamble of claim 1, for intensified breathing and to a valve, according to the preamble of claim 19, arranged in a patient conduit of an apparatus for intensified breathing.
Ventilators are used for intensifying the breathing of patients whose own breathing activity for some reason is inadequate. They are typically applied to patients anaesthetised and relaxated during surgery and to those in intensive care. A conventional ventilator provides a cyclic ventilation of lungs. The main phases of a cycle are inhalation and exhalation. A cycle recurs at a desired, controllable pace. A ventilator at its simplest is primarily in accordance with the preamble of claim 1, the needed pressure being provided e.g. by means of a blower.
Inhalation gas in high power ventilators is most often a mixture of oxygen and nitrogen, whereby an oxygen concentration can be arranged as desired between 21 to 100%. For the controlling of the mixture, ventilators are connected to two pressure tanks. The pressure in the pressure tanks is first brought to a constant level by pressure regulators. The bringing of the pressure to a constant level in the pressure tanks improves, on one hand, the adjustability of flow and reduces, on the other, the requirements set for inhalation valves used for controlling the gas flow in a particular conduit. The gas flows of the conduits are arranged to provide the above mentioned oxygen concentration in the mixture and a desired total flow. The total flow can be either a parameter determined directly by the operator or a desired flow value indirectly calculated by a control system from other parameter settings and/or measurement signals. The control system controls the inhalation valves by comparing a measurement signal proportional to the prevailing gas flow to the desired gas flow and by changing the control settings when needed. In the described system said measurement signal is a flow measuring element, but it can also be a location detector of a closing element of an inhalation valve, a pressure signal, a signal proportional to temperature, or a combination of these.
After the separate gas flows are measured and adjusted, the flows are combined and a gas mixture is produced. At this stage the pressure of the gas mixture measured from an inhalation conduit is in most cases, with sufficient accuracy, the same as it is when delivered to a patient. Said pressure can thus participate also in the determining of said desired gas flow, if the control system has been arranged to adjust a constant pressure delivered to the patient during inhalation. The pressure in the inhalation conduit is measured as a difference pressure in relation to the surrounding air pressure. Other functions included in a module comprise a measurement of the oxygen concentration of the mixture, which can, however, also be located closer to the patient.
From the gas mixture module the gas flow is conveyed along the inhalation conduit to a Y piece, to a second branch of which the patient is connected either through a breathing mask or an intubation tube, a third branch being connected to an exhalation conduit. Inhalation thus takes place by means of an over-pressurised gas mixture delivered from the pressure tank through the inhalation valve to the inhalation conduit, whereby the pressure in the inhalation conduit increases. An exhalation conduit is at the time kept closed by an exhalation valve, so the gas mixture is further conveyed to the lungs, causing also a rise in pressure there.
Exhalation takes place spontaneously when an over-pressure stored in the lungs during inhalation is released. This is achieved by the a closing of the inhalation valve(s) and by the opening of the exhalation valve in the exhalation conduit, whereby the pressure of the inhalation and exhalation conduits acting on the patient decreases. The over-pressure in the lungs is thus released as a gas flow through the exhalation conduit. The control system controls the exhalation valves and, when necessary, also the inhalation valves in such a way that the pressure level of an exhalation period is reached as quickly as possible and then kept as constant as possible. For adjusting the pressure of the exhalation period, the ventilator is often provided with a pressure sensor arranged in the exhalation conduit, although in most cases the pressures of the exhalation and the inhalation conduits come very close to each other.
Modern ventilators make full use of the opportunities offered by electronics for controlling a breathing cycle according to settings. The control is performed by arranging the inhalation and exhalation valves to implement parameter settings. These settings include inhalation over-pressure, exhalation over-pressure, inhalation volume and the duration of inhalation and exhalation efforts.
Since a ventilator is a life-supporting apparatus, it must be safe. This requirement for safety means that the apparatus must not cause danger to a patient or to a user during normal operation or in the case of a single malfunction. To ensure this, the ventilator is to be provided with safety circuits, which do not depend on its basic functions and which are activated when a basic function for some reason malfunctions. One of the most critical emergencies a ventilator can cause to a patient is over-pressurisation of the lungs. Over-pressure can damage the lungs in a very short time. Malfunction that can cause an over-pressure can occur e.g. when the exhalation valve is blocked to a closed position or the inhalation valve remains open. In order to prevent damages also in such cases of malfunction a restrictor valve is typically arranged in the ventilators. It is usually located in the gas mixture inhalation conduit.
Prior art comprises two types of functions restricting over-pressure. The more conventional one is a spring-loaded valve. The spring is calibrated to open at a predetermined constant pressure. A typical calibration pressure varies from 10 to 12 kPa. A constant limit pressure does not, however, guarantee a fully sufficient safety circuit, considering the varying needs of patients in intensive care. For patients in critical condition over-pressure can cause danger even before said limit pressures are attained. Another weakness in said safety valve is that it only releases an over-pressure higher than the calibrated pressure limit and in case of a malfunction the patient remains over-pressurised at said pressure limit, instead of the pressure dropping to the level of a normal exhalation pressure.
A more advanced version of this safety valve is to replace the calibrated spring with a control system. This control system keeps the valve closed either electronically or electro-pneumatically. An advantage the control system provides is that it allows the limit pressure to be set according to the patient's needs and, on the other hand, over-pressure to be fully released to the level of the exhalation pressure. The control system for an over-pressure valve comprises a pressure-sensitive element. A signal transmitted by this element is compared with a predetermined limit pressure. When the signal exceeds the limit pressure, the control system opens the restrictor valve of the over-pressure.
Another security feature associated with ventilators used particularly in intensive care is that they allow spontaneous breathing. Although the patients are connected to ventilators, they are often capable of breathing independently and they are allowed to do so. A breathing gas is usually mixed by inhalation valves with a desired oxygen concentration. Should the mixing system become damaged, the prior art solutions open the inhalation conduit to the environment, allowing spontaneous breathing to be continued from the atmosphere. The inhalation conduit is opened using either a separate valve or the function is combined with said over-pressure restrictor valve. Irrespective of the method of implementation, it is vital for the normal operation of the ventilator that the valve closes tightly when it is not needed to open, because any leaks would make the gas intended for the patient to flow out, which reduces the patient's breathing volume, causing other critical situations and leak alarms. The probability of leaks naturally grows as the number of opening components in the breathing conduits increases. The apparatus also needs to be cleaned after each patient and the accumulation after cleaning becomes more complex, increasing thus the probability of error.
A supplementary requirement associated with spontaneous breathing is to avoid return breathing, i.e. the mixing of exhalation and inhalation gases. Inhalation and exhalation gases are conveyed to different flow conduits by directional valves arranged in said conduits. In prior art solutions these valves are typically placed in the gas mixture module and in the exhalation valve module. The basic requirement set for these valves is small gas flow resistance to reduce the patient's breathing effort. In the inhalation conduit the directional valve is to be located between the patient and the point of opening of the inhalation conduit. In prior art systems with a combined opening for the inhalation conduit and the over-pressure restrictor valve said directional valve can cause danger when the directional valve prevents gas flow from the patient to the over-pressure restrictor valve, i.e. the function for which said valve is arranged in the apparatus.